9. Jupiter: a giant primitive planet

Stormy weather

Wind-blown zones and belts

The only features we can see on Jupiter are multi-colored clouds drawn into adjacent bands or stripes by the planetís rapid rotation. There are alternate dark bands, called belts, and light ones known as zones. The zones and belts surround the planet, running parallel to the equator at different speeds.

The giant planetís belts and zones are the sites of counterflowing winds that are sometimes called zonal, or east-west, jets. Where the Earth has just one westward air current at low latitudes - a trade wind, and one nearly eastward current at mid-latitudes Ė a jet stream, at the cloud top level Jupiter has five or six of these alternating jet streams in each hemisphere. Jupiter has five or six of these alternating set streams in each hemisphere.

Storm clouds

The colorful spots and stripes that dominate Jupiterís face mark patterns of stormy weather, as clouds billow, churn and seethe. Red, white and brown spots stare out of Jupiterís atmosphere like gigantic eyes. Huge storms larger than the Earth in size swirl across the planet, while smaller eddies chase each other whirling and rolling about.

Jupiterís famous Great Red Spot is essentially a huge weather system, with an east-west dimension greater than Earthís diameter. Because of rapidly increasing pressure with depth it cannot extend deeply into the planet. It is simply an enormous, shallow eddy trapped between counter-flowing jets, so large that the strong prevailing winds are forced to flow around it. The winds, in turn, funnel smaller eddies toward the Red Spot, helping to roll it around.

Cloud layers and colors

If we could descend through Jupiterís thin cloud layer, we would find that the temperature and pressure increase with depth. As in any planetary atmosphere, the atoms and molecules collide more frequently in the increasingly compressed, denser regions of Jupiterís atmosphere so the pressure and temperature increase there. At the cloud tops the temperature is a freezing 114 degrees kelvin and the atmospheric pressure is about 0.1 bar, or one tenth that of the Earthís air at sea level. In slightly deeper layers, about 130 thousand meters down, the temperature rises to a balmy 300 degrees kelvin, well above the freezing point of water, at 273 degrees kelvin. In these warmer regions, the pressure is comparable to the air pressure at the surface of the Earth, leading to speculations that, if verified mixing were small, living things might reside there. And above them it is cold enough to freeze various gases into ice to form the clouds.

The visible cloud tops of Jupiter consist of ammonia ice crystals, which condense out of the atmosphere at the very low temperatures and pressures there. They create graceful white clouds that probably make up the cold, light-colored zones observed from Earth. This is consistent with spectroscopic measurements of abundant ammonia at the cloud tops of Jupiter.

Lightning bolts in wet spots

Ancient mythology was close to the mark when it designated Jupiter as master of the rains, hurling thunderbolts at those who displeased him. Lightning flashes were discovered in Voyager 1 and 2 images of the dark night side of Jupiter, apparently illuminating the clouds in massive thunderstorms, and the lightning was confirmed by instruments aboard the Galileo spacecraft. Both missions showed that the lightning is concentrated near oppositely directed winds where storm clouds are found.

How deep the lightning occurs can be estimated from its diameter. The larger the flash, the deeper the lightning discharge. The observed sizes of Jupiterís lightning flashes suggest that they originate from layers in the atmosphere where water clouds are expected to form, at about 100 thousand meters down. Only water could condense at these depths. When the Galileo cameras followed the night-side lightning sources into the day side, they confirmed that the lightning originates in deep moist clouds.

Plunging into a dry and windy spot

A pioneering descent into Jupiterís atmosphere took place on 7 December 1995 when a 339-kilogram probe was dropped from the Galileo spacecraft on a suicide plunge into the planet. Scientists had expected that the Galileo Probe would pass through three cloud layers, composed of different chemicals that condense from tenuous gases at successively higher and colder levels. Below the bottom, water-cloud layer, which for Jupiter is formed at the 5-bar level, the atmosphere was expected to be well stirred, and therefore more representative of the planetís uniform, global composition. But contrary to expectations, the clouds were not where everyone thought they would be. It was apparently a clear day at the probeís entry point. When the capsule plummeted into the maelstrom, its instruments saw almost no evidence for clouds. Moreover, the planet was a lot drier than anticipated, at least in the vicinity of the probe-entry site. Extrapolating from the Sunís makeup, researchers had expected at least five or ten times as much water, under the assumption that Jupiter coalesced out of similar material with the same proportion of oxygen, which in the outer atmosphere of Jupiter should all be in the form of water molecules. Far less lightening was also detected during the probeís hour-long descent, supporting the conclusion that this part of the upper atmosphere contains little water.

The missing clouds and water might be explained if the probe descended into an unusually clear spot of dry, downwelling air and reduced cloud cover. In fact, observations from Earth-based telescopes indicated that the entry site was a region where internal shines through a gap in the clouds, and dry, wrung-out air from high altitudes might have been forced downward in there. Jupiter has several of these clear hot spots, that alternate with cloudy places in a band near the planetís equator.